Integrated electro-optical modulators are finding increasing use in state-of-the-art analog optical-fiber communication systems. Such systems are being developed as an alternative to conventional electromagnetic transmission lines. Present optical-fiber communication systems may consist of a high power, low noise laser source in conjunction with a wide bandwidth external modulator, for example. Such external modulators may be used in conjunction with high power, low noise lasers, to provide analog signal transfer in optical fiber cable T.V. networks. A common problem in such systems is that the dynamic range thereof is limited by intermodulation distortion, and by harmonic distortion due to modulator nonlinearities. Many attempts have been made in the art to improve the linearity of the modulators.
One method of improving the linearity of optical modulators is to provide electromagnetic predistortion of the input signal. A system for accomplishing this is reported in an article by R. P. Childs and D. A. O'Byrne, that appeared in OFC 90, entitled "Predistortion Linearization of Directly Modulated DFB Lasers and External Modulators for AM Video Transmission."
There are many examples of electro-optical modulators, demodulators, and modulating systems in the art. A number of such prior systems are briefly discussed below.
Johnson U.S. Pat. Nos. 5,015,053 and 5,002,355 refer to linearization of an interferometric modulator by coupling to transverse electric (TE) and transverse magnetic (TM) modes. A carrier wave derived from a light source is polarized by a polarizer for adjusting the power of the carrier wave in the TE and TM polarization modes.
Hamilton et al. U.S. Pat. No. 5,010,346 shows an electro-optical analog-to-digital converter. A laser light source is synchronized with a digital clock for producing pulses of light. The pulses of light are modulated by an optical modulator for varying the intensity of the pulses of light as a function of the amplitude of the analog modulating signal. The light pulses are demultiplexed and passed through a detector for converting the light pulses to electrical signals. The electrical signals are then passed through an electrical analog-to-digital converter.
DuPuy et al U.S. Pat. No. 4,997,245 shows optical switches that are polarization independent. The optical switches include a substrate of electro-optic material in which waveguides are formed in a manner permitting light waves to be switched therethrough independent of polarization phenomena.
Thylen U.S. Pat. No. 4,993,798 refers to an optical communication system consisting of a semiconductor laser that includes an optical waveguide coupled at one end to first and second optical directional switches. The other ends of each one of the directional couplers each have an end of a waveguide terminated to a mirror, for forming a cavity including the laser diode. By either modulating current driving the diode, or by applying a modulating signal to the directional couplers, a light signal passing therethrough can be modulated.
Yoon et al U.S. Pat. No. 4,936,645 shows a polarization-insensitive Mach-Zehnder modulator that includes a thin film waveguide on a substrate. The modulator consists of a lamination of different organic polymers upon which the thin film waveguide is deposited, and electrodes for applying voltages for producing electric fields to coact with the light passing through the waveguides.
Koai U.S. Pat. No. 4,934,776 teaches the combination of cascaded individual waveguide directional coupler sections arranged to provide a high-density optical data array, for forming an integrated optical intensity modulator. The cascaded coupled-waveguide modulators provide ultra-high extinction ratios.
Koai U.S. Pat. No. 4,934,775 shows an optical space-division switching architecture consisting of high-extinction electro-optical data arrays of a plurality of cascaded coupled-waveguide optical intensity modulators. A plurality of passive splitter circuits are used to evenly divide an input optical signal into a plurality of interim optical signals. The optical signals are passed through the optical intensity modulators, respectively, which have waveguide coupler sections cascaded together on a crystal substrate to form a bar-type modulator structure, for providing a direct optical connection between the various sections of each modulator. Combiner circuits receive the output optical signals from the data arrays and combine them for providing a single output wave or signal.
Su U.S. Pat. No. 4,932,736 teaches the monolithic integration or three integrated waveguide couplers to provide a 1.times.2 or 2.times.1 integrated optical switch with a high extinction ratio. In the switch a first waveguide coupler has its output directly coupled to the input ports of the other two waveguide couplers via an integrated waveguide. As a result, two cascaded integrated waveguide couplers are provided along each path from a single input port to two output ports, respectively.
Coleman U.S. Pat. No. 4,882,775 shows a demodulator provided by a an interferometer and a signal processor. The demodulator is included in a coherence multiplexed, optical data transmission system. The optical data transmission system includes an interferometric modulator at an input end. The output signals therefrom are modulated waveforms which are passed through an optical multiplexer, and coupled via single mode optical couplers and a fiber optic cable to demodulator stages. Each modulator and demodulator stage is provided by an unequal arm-length Mach-Zehnder interferometer. The multiplexers phase modulate the optical carriers from the modulators for multiplexing the same together. The path lengths between the modulators and demodulators included in the system are controlled for obtaining a desired output from the output demodulator stages 40.
Alferness U.S. Pat. No. 4,390,236 shows an electro-optical demultiplexer consisting of a plurality of cascaded tunable, polarization independent optical wavelength filters. The various sections of the filters, as cascaded, provide an input polarization selective coupler that separates the transverse electrical (TE) and transverse magnetic (TM) orthogonally polarized wave components, respectively, of an arbitrarily polarized input signal. The separated TE and TM components then travel along two different wavepaths or waveguides. Each waveguide includes a wavelength selective mode converter. An output polarization selective coupler is provided for combining in one output waveguide signal components centered about the wavelength of the mode converters, and for combining in a second output waveguide the balance of the input signal.
Papuchon U.S. Pat. No. 4,223,977 shows an integrated optical circuit for separating a plurality of optical waves of different wavelengths, thereby providing a demultiplexing function. A plurality of basic electro-optical cells are integrated on a single substrate for providing the optical demultiplexing circuit.
A paper entitled "Waveguide Modulators with Extended Linear Dynamic Range a Theoretical Prediction" (Z. Q. Lin and W. S. C. Chang, Photonics Technology Letters, Vol. 2, No. 12, Dec. '90) shows a modified-directional-coupler modulator with reduced nonlinear distortion. The coupler includes an input stage for receiving input signals for a voltage control directional coupler that couples the signals to a Mach-Zehnder interferometer. The output of the interferometer is coupled by a fixed directional coupler to an output waveguide providing two channels. The parameters are adjusted for minimizing third order intermodulation spurious signals.
A paper entitled "In Search of a Linear Electro-optic Amplitude Modulator" (Pao-Lo Liu, B. J. Li, and Y. S. Trisno, SPIE Conference, 1990) shows a linear electro-optic amplitude modulator. The modulator includes a Y-branch for dividing an optical input signal equally for inputting into the two arms, respectively, of a Mach-Zehnder interferometer. The interferometer functions as a phase shifter. The output of the interferometer is supplied to a directional coupler having a nominal coupling constant-length product of half a coupling length. The optical output signal is provided at each of two output waveguides. A modulation signal of one polarity is applied to the interferometer phase shifter, and is applied in reverse polarity to the directional coupler using a multiplication factor. The parameters are adjusted for optimizing the linearity of the modulator by reducing the second and third order harmonics that cause nonlinearities to occur.
The present inventor recognized that known integrated optics modulators require further improvement. One need recognized is to improve the linearity of integrated optical phase modulators.
In a paper entitled "Linearization Of Electro-Optic Modulators By A Cascade Coupling Of Phase Modulating Electrodes" (Halvor Skeie and Richard V. Johnson, SPIE Conference, Vol. 1583, Integrated Optical Circuits, Sept. 1991, p. 153) shows two configurations for linearized optical modulators for use in wide band analog transmission systems. The linearization by cascading of Mach Zehnder structures is shown.